A(1). Field of the Invention
The invention relates to a method and a system of transmitting digital information between a plurality of stations which are coupled to a unidirectional transmission ring. The ring comprises a monitor for generating a frame in the ring, in which frame a plurality of types of time slots can be distinguished, which types differ from each other in the number of bits which can be accommodated in the time slots. Time slots of the same type in consecutive frames providing together a corresponding number of numbered channels for transmitting the digital information between the stations.
Developments in communications techniques are characterized by an ever increasing diversification in the type of traffic to be processed. Since telegraphy, the communication service for written words, telephone, communication service for spoken words, and facsimile, the communication service for transmitting picture information there came into being in the last tens of years, parallel with the development of the computer, traffic: data between computers and between computers and computer stations. Partially the situation was that for each type of traffic a separate, often wire-commited, network was installed. In the last few years, a position has been made available in the existing services for new services. Thus, for the joint transmission of word and picture information the telephone line was utilized (videophone). The telephone line was also utilized for the transmission of computer data, for example for requesting and displaying data on a television screen at the subscriber (viewdata), or the inputting and outputting of data by means of a terminal for a remote computer.
As the possible services become increasingly numerous the need has recently arisen for networks which are capable of supporting a wide variety of these services. Starting point is the idea that in a geographically limited area, such as a factory, office, laboratory, university etc. an infrastructure is found which renders it possible to have access to that large variety of services. The networks proposed for that purpose, commonly denoted local area networks, are in essence formed by a transmission means, often a cable, which is laid as a fine mesh over the geographical area to be covered and over which stations can be connected to communicate with each other. A station may be a telephone set, a keyboard with picture screen, a computer, a word processor, etc. The digital information transmitted by the station over the transmission means may consequently consist of (a train of) digital speech samples or a data packet.
The proposed local area networks, LAN's for short, can be distinguished from each other by a number of features. In the first place, as regards topology of the transmission means, a distinction can be made between ring, bus and loop transmission means. A looped transmission means is obtained when a loop is formed from a central station through a large number of users stations back to the central station. With a bus means the stations are connected to the means which is terminated at both ends. In a bus and also in a ring transmission means the stations are interconnected without the existence of, or processing by, a central station. The characteristic feature of a ring transmission means is that the information, once it is present on the ring can circulate for an unlimited period of time without further measures as the ring behaves as a circulating memory. The transmission of new information is effected by overwriting a memory location in the circulating memory. A further distinguishing feature is the manner in which the stations get access to the means. This may occur in the "contention" or the "non-contention" mode. With a contention access strategy (such as, for example CSMA-CD) it may happen that more than one station transmits simultaneously over the means causing the messages of both stations to be mutilated. With a non-contention access strategy, (for example, in "token passing" techniques) it is always guaranteed that not more than one station can transmit at the same time. A further distinction is that the frequency band of the transmission means can be utilized in the baseband or in the broadband mode. The broadband mode is defined as the mode in which a plurality of different signals are simultaneously present on the means (for example by means of time or frequency-division multiplex) without these signals interfering with each other.
Finally, LAN's can be distinguished from each other according to the type (or types) of traffic capable of being supported by the local area network: full duplex circuit-switched traffic for supporting, for example, telephone and videophone traffic and/or packet-switched traffic for supporting data traffic.
The invention relates to a broadband, token passing, local area network ring, with which both circuit and packet-switched traffic can be supported.
A(2). Description of the prior art
The publication mentioned sub D(1) discloses a local area network ring in which the transmission capacity on the ring is made available for the stations connected thereto in the broadband, more specifically in time division multiplex. In addition, the ring comprises what is commonly referred to as a "loop synchronizer" which ensures that a frame structure is provided on the ring. For that purpose the loop synchronizer comprises a buffer which increases the propagation time of a signal on the ring until it has a duration of accurately one frame or a multiple thereof. The frame structure is arranged such that circuit-switched traffic, namely two 2 Mbit/s video conference channels and 48 telephone channels of 64 kbit/s each and message-switched traffic, namely one data link between 1 Mbit/s computers can be supported. The access to the transmission means (optical fibre) for the circuit-switched traffic is regulated by means of tokens which are circulated through the ring token channels therefor. Each circuit-switched channel has its own token channel by means of which the access to that channel is regulated.
In addition, the loop synchronizer provides that the beginning of a frame is marked, which renders it possible to identify the channels in the frame by counting from the initial mark.
Such a system has the disadvantage that reserving an information channel of a given category implies that in principle all the token channels of the relevant category must be inspected. This imposes either high requirements on the processing rate of the station if all channels of each passing frame must be monitored, or the time required to obtain access becomes comparatively long if the station monitors a token channel only once per passing frame, so that the transmission means is not utilized optimally.